Previous Next Contents

Module Gc: memory management control and statistics

type stat = {
  minor_words : int;
  promoted_words : int;
  major_words : int;
  minor_collections : int;
  major_collections : int;
  heap_words : int;
  heap_chunks : int;
  live_words : int;
  live_blocks : int;
  free_words : int;
  free_blocks : int;
  largest_free : int;
  fragments : int;
  compactions : int
The memory management counters are returned in a stat record. All the numbers are computed since the start of the program. The fields of this record are:
minor_words Number of words allocated in the minor heap.
promoted_words Number of words allocated in the minor heap that survived a minor collection and were moved to the major heap.
major_words Number of words allocated in the major heap, including the promoted words.
minor_collections Number of minor collections.
major_collections Number of major collection cycles, not counting the current cycle.
heap_words Total number of words in the major heap.
heap_chunks Number of times the major heap size was increased.
live_words Number of words of live data in the major heap, including the header words.
live_blocks Number of live objects in the major heap.
free_words Number of words in the free list.
free_blocks Number of objects in the free list.
largest_free Size (in words) of the largest object in the free list.
fragments Number of wasted words due to fragmentation. These are 1-words free blocks placed between two live objects. They cannot be inserted in the free list, thus they are not available for allocation.
compactions Number of heap compactions.

The total amount of memory allocated by the program is (in words) minor_words + major_words - promoted_words. Multiply by the word size (4 on a 32-bit machine, 8 on a 64-bit machine) to get the number of bytes.

type control = {
  mutable minor_heap_size : int;
  mutable major_heap_increment : int;
  mutable space_overhead : int;
  mutable verbose : bool;
  mutable max_overhead : int;
  mutable stack_limit : int
The GC parameters are given as a control record. The fields are:
minor_heap_size The size (in words) of the minor heap. Changing this parameter will trigger a minor collection. Default: 32k.
major_heap_increment The minimum number of words to add to the major heap when increasing it. Default: 62k.
space_overhead The major GC speed is computed from this parameter. This is the memory that will be wasted because the GC does not immediatly collect unreachable objects. It is expressed as a percentage of the memory used for live data. The GC will work more (use more CPU time and collect objects more eagerly) if space_overhead is smaller. The computation of the GC speed assumes that the amount of live data is constant. Default: 42.
max_overhead Heap compaction is triggered when the estimated amount of free memory is more than max_overhead percent of the amount of live data. If max_overhead is set to 0, heap compaction is triggered at the end of each major GC cycle (this setting is intended for testing purposes only). If max_overhead >= 1000000, compaction is never triggered. Default: 1000000.
verbose This flag controls the GC messages on standard error output. Default: false.
stack_limit The maximum size of the stack (in words). This is only relevant to the byte-code runtime, as the native code runtime uses the operating system's stack. Default: 256k.

val stat : unit -> stat
Return the current values of the memory management counters in a stat record.
val print_stat : out_channel -> unit
Print the current values of the memory management counters (in human-readable form) into the channel argument.
val get : unit -> control
Return the current values of the GC parameters in a control record.
val set : control -> unit
set r changes the GC parameters according to the control record r. The normal usage is:
       let r = Gc.get () in    (* Get the current parameters. *)
         r.verbose <- true;    (* Change some of them. *)
         Gc.set r              (* Set the new values. *)

val minor : unit -> unit
Trigger a minor collection.
val major : unit -> unit
Finish the current major collection cycle.
val full_major : unit -> unit
Finish the current major collection cycle and perform a complete new cycle. This will collect all currently unreachable objects.
val compact : unit -> unit = "gc_compaction";;
Perform a full major collection and compact the heap. Note that heap compaction is a lengthy operation.

Previous Next Contents